An international questionnaire survey is underway, aimed at obtaining standardized information to provide insight into many presently unanswered questions as to clinical features of homocystinuria due to cystathionine Beta-synthase deficiency and the effects of various therapies now in use for this condition. Data collection is now virtually complete. Information upon more than 630 patients has been received and entered into computer format. Analyses of the data are now underway. Although final results are not yet available, it is certain that the study will accomplish its chief aims: to provide base-line data for untreated patients with respect to mental retardation, thromboembolic episodes, lens dislocation, survival, and appearance of osteoporosis. These data will provide the necessary means to evaluate therapies as data accumulate in the future.